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concluded in September, 2009. Please visit NASA at http://nasascience.nasa.gov/
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For nearly a century, we've known that the universe
is expanding: Distant galaxies are moving further away from us
and from each other. Until recently, scientists thought this expansion
must be gradually slowing down, as gravity put on the brakes. But
the big surprise of the past decade was the discovery that
the expansion of the universe is speeding up! Something is
driving the universe apart. Scientists do not yet understand
the source of this acceleration, but they have given it a name:
dark energy. Whatever it is, it appears to make up most of
the mass of the universe.

How do we know that the expansion of the universe is speeding
up?
After all, even through the most powerful telescope, no astronomer
can see galaxies move directly, even watching for an entire lifetime.
It's a bit like looking at a car that is so far down the highway
that we can't see whether it is moving at all. Fortunately, we
can tell how fast galaxies are moving away from us by analyzing
the light coming to us from the galaxy. The method is related to
one used by traffic police to tell how fast a car is moving.

There's another, more difficult, problem as well. The expansion
of the universe does not change fast enough for us to detect over
our lifetime. We can only tell if the universe is speeding up by
comparing its expansion today with how fast it was expanding in
the distant past.

You might think it would be impossible to tell what the universe
was doing in the past; no one was there to observe it. But amazingly,
wonderfully, nature gives us a way to see the past directly. The
universe is so large, that it takes a very long time for light
to reach Earth from distant galaxies. Therefore, the image that
we capture today shows us what the object looked like in the past,
when light left it - not what it looks like now, when we are receiving
that light. By observing the motions of galaxies at different distances,
astronomers can tell how fast the universe was expanding at different
times in the past.

A key breakthrough was finding a technique to determine the distance
to a galaxy, and therefore to tell how far back in the past we
are looking. The technique rests on the happy accident that when
a certain type of star dies, it explodes with a spectacular flash
whose inherent brightness is known. These exploding stars, called
supernovae, enable astronomers to determine the distance to the
star and the galaxy in which it lives and dies: The dimmer the
flash appears, the further away the star must be. In its death,
a star is a beacon to all observers throughout our universe, marking
out distance in the dark reaches of space.

From all this information, astronomers have been able to piece
together how fast the universe was expanding at different times
in its history. The result is clear: Sometime around 5 billion
years ago, the universe began accelerating - its expansion getting
faster and faster, rather than gradually slowing down. The conclusion:
Something, which scientists now call dark energy, must be driving
the acceleration of the universe. The dark energy was presumably
there since the beginning of the universe, but its effect becomes
more important as the universe expands.

Astronomers are able to determine how much dark energy there is
as well, based on the measured rate of expansion of the universe.
(They are able to do this because Einstein's theory of gravity
relates the expansion rate of the universe to the total amount
of all forms of mass or energy in the universe.) The result is
that about 70% of the content of the observable universe must be
dark energy. In other words, most of the universe is made of some
mysterious form of energy whose nature is completely unknown.

Important conclusions should be based on more than a single line
of evidence. Astronomers now have other independent lines of evidence
that confirm the reality of dark energy. Now a new and extraordinary
chapter begins: the search for the nature of dark energy. Join
the search in the next
section >